Counting Starspots

The Sun has been pretty quiet the last few years. It’s on the downward slope of its 11-year magnetic cycle, so only a few dark sunspots have marked its surface. The numbers should stay low awhile longer, until the cycle heads toward its peak in the next decade.

Astronomers have measured magnetic cycles in quite a few other stars. And they’ve found one star with a cycle that appears to be a little longer than the Sun’s — more than 11 years. And thanks to an orbiting planet, they’ve even been able to map its starspots, which show a pattern similar to the Sun’s.

HAT-P-11 is a bit smaller, cooler, and redder than the Sun, and it takes about the same amount of time to spin on its axis.

It’s orbited by a planet that’s about the size of Neptune, one of the giants of our own solar system. The planet crosses in front of the star every few days, blocking some of the star’s light. Observations by telescopes on the ground and in space found that the amount of starlight that’s blocked varies by a tiny amount. That’s because the planet sometimes passes in front of the dark starspots, which don’t add much to the star’s overall brightness.

By tracking the planet through 200 crossings, astronomers mapped hundreds of starspots. They found that the star has more spots than the Sun, and many of them are bigger than typical sunspots. But they appear at the same latitudes as the spots on the Sun, giving HAT-P-11 some dark bands on either side of its equator.

Script by Damond Benningfield